"Some older studies suggest that taxes on sugar-sweetened beverages will reduce obesity by 20 percent rely on household data instead of individual consumption patterns, and they assume that individuals don't replace the calories in the soda with calories from another source," says Fletcher, who co-authored the article with David E. Frisvold, of the University of Iowa's Economics Department, and Nathan Tefft, of the University of Washington's School of Public Health. "In contrast, our study found that increases in soft drink tax rates do correlate to less soda consumption, but not a reduction in calorie intake."

In their new research, published initially online by the journal Health Economics in March, Fletcher and his co-authors conducted two studies. One uses National Health and Nutrition Examination Surveys data to estimate effects on reported consumption and caloric intake of soda and other beverages as well as measured height and weight for a nationally representative sample of adults between 1989 and 2006.

"This evidence demonstrates that large increases in soft drink taxes are unlikely to reduce total caloric intake," Fletcher says. "The impact of soft drink taxes on the body mass index is small in magnitude and not statistically significant."

The second study looks at Ohio and Arkansas in the early 1990s. Both states substantially increased soda taxes. Fletcher, Frisvold and Tefft compare weight outcomes in those two states to outcomes in control groups drawn from other states. The apparent effects of a soda tax depend on which control group they used.

Additionally, although the tax appears to reduce body mass index and obesity prevalence in Arkansas when compared with states with no tax change, the tax increased body mass index in Ohio, Fletcher says.

"Our results cast serious doubt on the assumptions that proponents of large soda taxes make about the effects on population weight," Fletcher says. "Given that people substitute other calories when they give up soda, these new results suggest we need fundamental changes to policies that make large soda taxes a key element in the fight to reduce overall obesity rates."